1. Field of the Invention
The present invention relates to a hot-melt ink for use in ink-jet recording systems.
2. Related Art of the Invention
Ink-jet recording systems include electric field-controlled systems wherein electrostatic attraction is employed for ink ejection; drop-on-demand systems (pressure pulse systems) wherein the vibrational pressure provided by piezo elements is employed for ink ejection; and thermal ink-jet recording systems wherein-the pressure produced by bubbles formed and expanded using high heat is employed for ink ejection. These systems all afford printed images of high detail.
The inks employed in such ink-jet recording systems are ordinarily water-based inks, in which water is the principal solvent, and oil-based inks, in which an organic solvent is the principal solvent. Images printed using water-based inks on the whole have inferior water resistance; in contrast, using oil-based inks it is possible to produce printed images exhibiting excellent water resistance.
Since these water-based inks and oil-based inks are liquid at room temperature, feathering tends to occur as images are printed onto recording paper by use of these inks, and an adequate print density can not be obtained. Because these inks are liquids, they tend to cause the formation of deposits from the inks, whereby the reliability of ink-jet recording systems becomes degraded.
Hot-melt inks, which are solid at ambient temperature, have been proposed as a means for overcoming the drawbacks of the inks pertaining to the prior art, which are liquid at room temperature. U.S. Pat. No. 3,653,932 teaches an ink containing a dialkyl ester of sebacic acid. U.S. Pat. No. 4,390,369 and Japanese Patent Application Laid-Open No. 58-108271 teach inks containing natural waxes. Japanese Patent Application Laid-Open No. 59-22973 teaches an ink containing stearic acid, while Japanese Patent Application Laid-Open No. 61-83268 teaches an ink containing an acid or alcohol with 20 to 24 carbon atoms and a ketone having a relatively high melting point. Japanese Patent Application Laid-Open No. 62-48774 teaches an ink containing a thermosetting resin that has a high hydroxyl value, a solid organic solvent with a melting above 150.degree. C., and a small quantity of dyestuff. Japanese Patent Application Laid-Open No. 2-167373 teaches an ink containing coloring matter, a first solvent that is solid at room temperature and that liquefies when heated to a temperature above room temperature, and a second solvent that dissolves the first solvent and that is a highly volatile liquid at room temperature, Japanese Patent Application Laid-open No. 62-295973 teaches an ink containing a synthetic wax that has polar groups and a dye which is soluble in this synthetic wax. Japanese Patent Application Laid-Open No. 2-29471 teaches an ink containing ketones, amide waxes, waxes, and coloring matter.
Hot-melt inks are heated to a given temperature in the ink reservoir, recording head, and other components to produce a molten state exhibiting the desired viscosity, about 5 to 20 cps. In ink-jet recording systems, the viscosity is selected with reference to parameters such as the ink jetting hole diameter and the distance of the recording head from the recording paper. Accordingly, a drop in viscosity over time can result in satellite production, while a rise in viscosity can result in increased drop diameter, and the quality of the printed image is impaired. Thus, it is necessary that the viscosity of the molten ink not fluctuate significantly over extended periods. The hot-melt inks of the prior art, however, have poor thermal storage properties, and are susceptible to changes in viscosity.